The present invention concerns an improved pore water pressure meter of the kind used to measure the water pressure inside masses of earth or clay.
Pore water pressure meters of various kinds already exist to measure the water pressures existing between the granules in earth or clay. This water pressure, the pore water pressure, is of particular importance to the capacity of clay layers to support loads, e.g. of buildings.
One kind of pore water measuring devices presently in use is provided with a diaphragm one side of which is affected by the pore water pressure. At the opposite side of the diaphragm is attached a steel wire string. The latter is under a certain tension and extends axially through a tubular body which forms part of a rod by means of which the pore water pressure meter is pressed into the ground. The opposite end of the steel string is secured to a nipple in the rod. When the diaphragm is pressed inwards by the water pressure, the tension in the string is reduced. Consequently, the string becomes less tightened and as a result hereof its natural frequency, i.e. the frequency with which the string oscillates when set in motion, is reduced. The string is kept oscillating by means of two electro-magnets provided with coils connected to a tone frequency oscillator the frequency of which thus is determined by the natural frequency of the string. The oscillator is usually incorporated in the pore water pressure meter. The meter is lowered together with the rod to the location in the mass of earth where one intends to measure the pore water pressure. To prevent earth and stones from reaching the diapgragm the latter is protected by a filter allowing passage-through of water only.
The pore water pressure meter is calibrated by allowing a known pressure to act on the diphragm whereafter the string frequency is read. The relationship between frequency and pressure thus is obtained in the form of a curve. When the pressures on both sides of the diaphragm are equal, the string oscillates at a certain frequency, denominated zero pressure frequency. When the pressure acting on the diaphragm increases, the frequency is reduced. The larger the discrepancy between the established frequency and the zero pressure frequency, the larger evidently the pressure acting on the diaphragm.
In case of excess loading of the diaphragm - as a result of vibrations and thrusts or through plastic deformation - the zero pressure frequency may, however, change. If this happens after insertion of the pore water pressure meter into e.g. an earth dyke, conventional pore water pressure meters offer no possibility of calibration thereof or of establishing the new zero pressure frequency. The results obtained in the measuring operation on the whole therefore become useless. In most instances it is impossible to know whether or not the zero pressure frequency is altered but when there are reasons to suspect that this is the case, the measuring results obtained from the pressure metered cannot be considered reliable.